1. Field of the Invention
The present invention relates generally to high strength foam filled materials. More particularly, the present invention involves superior strength, uniform density polycyanurate foam filled honeycomb materials and methods for their production.
2. Description of Related Art
Light weight materials having a uniform cell structure are widely utilized in structurally and functionally demanding applications. By incorporating rigid foams within the cell structure of these materials, their compressive strength can be dramatically increased without severely affecting the density of the material. Foam filled honeycomb shaped structures, in particular, have wide utility as light weight insulation and structural materials in a variety of industries including the building and aerospace industry.
Frequently, the environmental conditions to which structural materials are exposed include high temperatures and high humidities. Thus, rigid foams incorporated in honeycomb materials exposed to these extreme conditions must be stable to elevated temperatures and resistant to moisture absorption and hydrolysis. Moreover, these foams should be resistant to fire and not be self-igniting at elevated temperatures. Additionally, methods for fabricating foam-filled honeycomb parts often require shaping the honeycomb using machining and cutting techniques. Thus, rigid foams which do not structurally withstand machining procedures are not suitable candidates for incorporating into honeycomb.
An important property to consider in evaluating the structural integrity of foam-filled honeycomb structures is the uniformity of the foam density. The process used to incorporate the foam within the cell preferably produces a foam having highly uniform and fine foam cell density with a strong adhesive bond between the foam and the cell walls.
Polymethacrylimide foamable precursors are foamed from a highly viscous composition. This foaming process is a three dimensional expansion which often results in damage to the honeycomb structure. When the polymethacrylimide is present as a viscous composition it does not wet the honeycomb walls during the foaming process and consequently does not form strong bonds with the honeycomb.
Another method for incorporating foam into honeycomb structures is known as a powder process in which a foamable powder is distributed evenly on a substantially flat surface and the honeycomb structure is then pressed into the powder. By adding heat to the foamable powder and honeycomb structure, the powder foams and fills the honeycomb cells. This powder blowing method typically results in foams having an uneven density distribution exemplified by foams with cell sizes which vary from one honeycomb cell to the next. This density problem is generally attributed to difficulties in forming an even distribution of foamable powder prior to initiating the foaming process.
Accordingly, it is an objective of the present invention to provide foam filled honeycomb structures having reduced moisture sensitivity and improved strength over prior art foam filled honeycomb. It is also an objective of the present invention to provide foam filled honeycomb having highly uniform and controlled foam density with superior bond adhesive strength between the foam and honeycomb cell walls.